Solar collectors comprising a collector plate upon which a grid of fluid conduits is disposed in efficient heat-conductive relation and disposed in spaced relation beneath one or more transparent covers have been used to collect solar energy for space and hot water heating as well as air conditioning.
Some prior art collectors of this nature may be mounted in a frame or enclosure with sufficient insulation to minimize heat losses. Such units may be conveniently mounted on a roof or other supporting structure so as to be subjected to solar radiation. Other collectors are designed to be integral with the roof structure of a house or other building, in which case the collector plates may be mounted between the rafters, and the transparent cover is attached to the top edges of the rafters in covering relation to one or a plurality of collector plates. In such installations insulation is applied at the back of the collector plate between it and the attic space.
In normal operation of solar collectors of this type, fluid such as water is made to flow through the grid of conduits which, as stated above, are in good thermal contact with the collector plate. The collector plate is made efficient for solar energy adsorption such as by painting it black, and as a result of such absorption the temperature of the collector plate will increase and consequently the water flowing through the conduits will be heated. The heated water can then be used for space or hot water heating, as desired.
It has been found that if the flow of fluid is reduced the incremental temperature of the fluid, as it passes through the grid of conduits, will increase. If the flow of fluid should cease, the collector plate, conduits, and fluid within them will heat up to a temperature limited by the re-radiation from the collector plate. In present solar energy collector designs the collector plate, with no fluid flow or with conduits empty of fluid, can reach temperatures of more than 350.degree. F (about 177.degree. C).
Such high temperatures place extreme thermal stress on the components and materials of which the solar collectors are made. This may arise, for example, from the different thermal expansion coefficients of the materials used, resulting in breakdown of bonds and joints and outgassing of the materials in the collector. In addition, if the fluid in the conduits vaporizes, the resultant pressure may be sufficient to force leaks or even rupture or explode the conduits and joints. Excessively high temperatures for lengthy periods of time may also break down plastic materials and wood, and may even result in charring or burning of adjacent combustible materials.
Excessive heating of a solar collector plate can arise from many system failures. These include electrical power outage, pump failure, failure of circulation systems caused by leaks or blockage or failure of the control system. Since a serious or even catastrophic failure of a solar collector due to excessive heating could result from any of the above failure modes, it becomes important to build in a protective system for the collector to prevent overheating.